Installing fast:
Copy your current 7.1.5 directory to somewhere else.
Download the Battle.net installer / WoW installer
Point the Blizzard WoW installer to the copy of 7.1.5
Download the Firestorm PTR archive appropriate for your platform (Windows or Mac)
When the WoW installer is done, unpack the Firestorm PTR archive to the WoW directory
Run the PTR executable (-64 if you have a 64 bit OS).
Keep your main 7.1.5 WoW install to so you can keep playing on Sylvanas.
Some common problems:
Problem: 0% CPU using WoW instance when running the PTR executable in your 7.1.5 directory Solution: Install WoW 7.3.5 somewhere, unpack the Firestorm PTR archive to that directory, run the proper PTR executable from that directory.